The present invention relates to a data disk storage system utilizing a stepper motor for controlling the movement of a read/write head across the tracks of the data disk.
In the operation of stepper motors, otherwise sometimes referred to as impulse motors, the rotor of the motor is incrementally moved through a series of discrete movements or steps as a result of a corresponding number of discrete changes in the energization of the windings of the stator of the motor. By varying the current supplied to the stator of the motor, an incremental drive force is generated for driving the rotor in the series of incremental steps. The energy supplied to the stator windings creates an internal magnetic field which generates a torque that urges the rotor to assume a mechanical position in line with the resultant magnetic field. By changing the energization current supplied to the windings of the stator in sequential step-by-step fashion the magnetic field is caused to move in a similar step-by-step fashion thereby resulting in the incremental step-by-step movement of the rotor. In a simple motor, one full rotation of the rotor may result from 360 electrical degrees of rotation with 4 steps being required to achieve such rotation.
In a conventional 2-phase permanent magnet or hybrid stepper motor used in conjunction with disk drive systems, a control mechanism commonly has been utilized for doubling the possible number of discrete positions or steps, that the output shaft can adopt. This doubling operation is accomplished by alternately passing current through both windings simultaneously and then through only one winding. The angle through which the motor shaft moves for each of these current changes is equal to one half of one motor step. Examples of such half step control systems are disclosed in U.S. Pat. Nos. 3,077,555 to Fredrickson and No. 3,746,958 to Leenhouts.
While this technique provides a simple way of increasing the resolution, i.e. number of steps during each cycle of the stepper motor, it causes the following disadvantages:
(a) The holding torque on the control arm, for insuring that the read/write head is properly aligned with a selected data track, in the "half step" position is reduced because of the lower total quantity of current being supplied to the motor. (In the half step position only one winding is energized.) As a result, the step accuracy is poorer in the half step position since less torque is available to overcome frictional forces. In addition, the decrease in stiffness, i.e. holding torque, reduces the resistance of the rotor to movement under conditions of shock and vibration. PA1 (b) The accuracy of the motor cannot be controlled by the driving circuit in this half step position since any change of motor current in the half step will result in movement of the rotor but not in any change in the holding torque and stiffness.
While these disadvantages may be tolerable in some situations, they result in undesirable positioning problems in high performance applications. These problems become even more significant when seeking to minimize the separation between data tracks in a high density data storage system.
Numerous other stepper motor control systems have been developed primarily for increasing the number of steps within each motor cycle and additionally some attempt has been made for developing sophisticated control systems for overcoming the problems associated with the unequal torque applied to the rotor during the stepping operation. Exemplary of such developments are the various stepper motor control systems disclosed in those U.S. patents discussed below.
U.S. Pat. No. 3,445,741 to Gerber discloses a stepper motor in which each full step is broken down into a large number of smaller or fractional steps by varying the energization of the stator winding between a number of levels, which includes one or more levels in addition to 0,+1 unit levels. The control unit utilized in conjunction with the stepper motor disclosed by this patent to Gerber controls the current flowing through each of the stator windings for providing the additional current level.
U.S. Pat. No. 3,800,206 to Hinachi et al. discloses a drive control system for a 4-phase stepping motor. In the operation of the system disclosed in the patent to Hinachi et al. the number of steps for the rotor is increased by alternately energizing one coil and then a pair of coils of the stator windings. Uniformity of the resulting torque which would be inherent with such an energization system is avoided by increasing the exciting current during a time when one phase coil along is excited so as to be larger than the exciting current per phase during a time when 2-phase coils are excited.
U.S. Pat. No. 3,728,598 to May discloses a stepper motor having stator windings connected into two phases, with the stepper motor being advanced one step by reversing the direction of current in one of the phases. An energization circuit for this stepper motor continually energizes both portions of the windings so as to enable the available torque of the motor to be increased for at least the lower stepping speeds without increasing the losses in the motor.
U.S. Pat. No. 3,787,727 to McSparran discloses a half step stepper motor control system in which the acceleration and deceleration periods are held constant independent of the starting and stopping operations of the first and second phase energization signals. As pointed out by this patent, it is desirable to operate a motor in a half step manner in order to achieve twice the number of stable step positions. However, during such half step operation the torque applied to the motor varies in accordance with whether one or two windings are energized. The acceleration imparted to the rotor during the stepping operation will vary as a result of the number of windings energized and these different starting conditions will result in different lengths of time for the motor to arrive at its normal running speed unless appropriate steps are taken. Similar problems occur in connection with the deceleration of the half step operation of the stepper motor. The problems of start-up caused by the variation in torque in the stepper motor is compensated for in accordance with the disclosure of this patent by varying the current in the motor windings during start-up dependent upon whether it starts in 1-phase or 2-phase and the problems of stopping the motor caused by variation in torque is compensated for by changing the rate of the step pulses as well as the current amplitude in the windings during the stopping time dependent upon whether the motor is to stop in 1-phase or 2-phase.
U.S. Pat. No. 3,885,210 to Burnett discloses a stepper motor having a plurality of windings that are energized in sequence in order to generate the desired stepping movement of the motor. The drive circuit varies the current flowing into each of the windings in incremental steps between 0 and a maximum value in order to produce equal incremental steps of the motor.
U.S. Pat. No. 4,087,732 and No. 4,140,956 to Pritchard disclose a control circuit for a stepper motor for rotating the rotor of the motor in such a manner so as to minimize motor resonance while not appreciably reducing the rotor torque at high speeds. In the operation of this stepper motor, all of the steps are of equal amplitude but at least one of the steps between the maximum and minimum levels has a duration which is unequal to that of another step. In order to meet the requirement of maintaining the motor steps of equal size and maintaining equal holding torque, at least two of the components of the motor's stator that induce the magnetic fields are continuously changed.
U.S. Pat. No. 4,283,672 to Throssell discloses a stepper motor drive for sequentially energizing the windings of the motor and having an adjustable feedback network for controlling the operation of the current supplied to the windings. The drive includes at least one generator with a staircase signal that has a multiplier fed by a reference signal and an output of a clocked binary counter. The adjustable feedback network combines a fraction of the output of the multiplier with a reference signal so that the output from the multiplier represents unequal steps of a staircase wave form. A switching network which is controlled by the counter applies the drive signals to the windings in accordance with the output of the multiplier.